Studying gas sensing effects in n-In 2 O 3 films, we have observed the anomalous behavior of reducing gases such as CO and H 2 . At T oper <250–300 °C the decrease of In 2 O 3 film conductivity took place during the interactions with CO and H 2 . Such effect is specific for n-In 2 O 3 interactions with oxidizing gases. Acceptor-like behavior of conductivity has been observed in humid atmosphere only. The In 2 O 3 films were deposited by spray pyrolysis from InCl 3 –water solutions. Their thickness was varied between 20 and 400 nm. The maximum of acceptor effect took place for In 2 O 3 films with minimal thickness and minimal grain size. The mechanism of CO and H 2 interactions with chemisorbed molecular oxygen has been proposed to explain of the observed effect. According to this model, the low temperature CO and H 2 oxidation by molecular oxygen is followed by accumulation of atomic oxygen forms (O) on the surface of In 2 O 3 . Water plays a role of catalyst in these reactions. Mechanism of such influence is connected with formation of intermediate complexes on the surface of In 2 O 3 .
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Korotcenkov et al. (2004) studied this question.